1. Field of the Invention
The present invention relates to a magnetic toner conveying apparatus used with an electrophotographic image forming apparatus, and an electrophotographic image forming apparatus using game.
An "electrophotographic image forming apparatus" is an apparatus for forming an image on a recording medium using an electrophotographic process, such as an electrophotographic laser beam copying machine, an electrophotographic facsimile system, an electrophotographic printer, an electrophotographic word processor and the like. Incidentally, an "electrophotographic process" means that an electrostatic latent image is formed on an electrophotographic photosensitive member using a photoelectric phenomenon and the electrostatic latent image is visualized by electrostatically adhering toner to the latent image.
2. Related Background Art
In known electrophotographic image forming apparatuses, magnetic toner has been widely used for image formation. In such image forming apparatuses, an electrostatic latent image formed on a photosensitive member is developed with magnetic toner as a toner image (visualized image), and the toner image is transferred onto a recording medium such as a paper sheet. Thereafter, the toner image on the recording medium is permanently fixed to the recording medium by heat and pressure. However, all of the toner used for forming the toner image is not necessarily transferred to the recording medium; a little amount of toner remains on the photosensitive member. Thus, in order to always to obtain a high quality image, whenever a transferring operation is finished, it is necessary to remove residual toner remaining on the photosensitive member. In the past, it has been proposed that the waste or re-used toner is returned to a developing means through a convey means for re-use of the waste toner. For example, the residual toner remaining on the photosensitive member is removed by a cleaning device, and the re-used toner removed from the photosensitive member is returned to the developing means by a convey screw arranged in a return pipe and adapted to convey the toner discharged from the cleaning device to the developing means.
A toner conveying ability of a convey screw will be explained with reference to FIG. 6. The convey screw for conveying the toner is formed by (i) arranging a rigid shaft at a central portion of a spirally formed screw or (ii) forming an elongated plate in a spiral shape. As shown in FIG. 6, when the toner is conveyed in a horizontal direction by the screw of the above (i) type, as shown, as the convey screw is rotated in a clockwise direction (viewed from the right in FIG. 6), the toner T adhered to the screw blades is conveyed to the left due to the drag of the blades. In this case, the following forces act on the toner T.
When a mass of the toner is M, a coefficient of friction between the toner and the screw blade is .mu., an angle between the screw blade and a vertical direction is .theta. and the gravitational acceleration is g, the toner is subjected to a friction force of .mu.Mgsin.theta. generated by friction between the toner and the screw due to drag of the screw, and a component (Mg cos.theta.) of the gravity force Mg along a direction of the screw blade.
In this case, if the following relation (1) is satisfied, the toner is dropped along the screw blades to be stored at a lower portion of the convey path so that the toner can be conveyed by the drag of the screw in the conveying direction. EQU Mg cos.theta.&gt;.mu.Mg sin.theta. (1)
On the other hand, if the following relation (2) is satisfied, the toner is rotated together with the screw blades while being adhered to the blades, thereby applying no conveying force to the toner, with the result that the toner is trapped at a certain position, thereby causing blocking due to poor conveyance. EQU Mg cos.theta.&lt;.mu.Mg sin.theta. (2)
In order to avoid this, it has been proposed to reduce the coefficient of friction between the toner and the screw surface by coating the surface of the screw with Teflon. Further, the conveying force obtained by the convey screw of the above (ii) type is substantially the same as that of the convey screw of the (i) type.
By the way, since the screw of the (i) type has the shaft, there is a danger of decreasing the conveying force due to adhesion and solidification of toner around the shaft, and, since the shaft is rigid, the shaft of this kind is hard to be arranged in a curved convey path.
In the screw of the above (ii) type, since there is a space in a central portion of the screw, the toner rotated together with the screw blades can enter into the space and be dropped downwardly during the rotation of the screw, thereby permitting the conveyance of the toner. However, since a screw of this kind cannot maintain its original configuration due to its flexibility, it cannot be used with a convey path which is opened along a circumferential direction by half or more. On the other hand, in order to solve this problem, when the toner is magnetic, it has been proposed that the screw is made of magnetic material and a magnet is arranged at a lower portion of a convey path enclosing the screw, whereby the toner is forcibly gathered at a lower portion of the convey path so that the screw of the (ii) type can be used with a convey path which is opened at its upper half (see Japanese Utility Model Publication No. 4-52774).